1. Field of the Invention
The present invention relates to an improvement of an auxiliary signal transmission system used, for example, in a digital microwave multiplex radio communication system.
To carry out the transmission of a digitized order signal, low speed data or a system switching control signal (generically known as the auxiliary signal) between the terminal office and the intermediate repeater station, between the intermediate stations, and between the terminal offices, by using the digital microwave multiplex radio system, the system is used for inserting these signals into a main signal (sometimes known as a digital service channel and hereinafter referred to as DSC), and in the intermediate repeater station, signal processing is carried out in a signal processing portion, to take out or repeat the order-wire signal or the low speed data from the received signal. In this case, desirably the auxiliary signal transmission system prevents problems in the system even if a problem occurs in the intermediate repeater station.
2. Description of the Related Arts
In a digital microwave multiplex radio system which transmits the auxiliary signal between the terminal office and the intermediate repeater station, or between intermediate repeater stations, if the transmission capacity is small, a system which performs a shallow frequency modulation of a carrier wave by this signal, to carry out an analogical transmission (such a system is known as an analog service channel) is used. However, for example, when the transmission capacity is large, such as in a digital multi-value quadrature amplitude modulation system, the purity of the regenerated carrier wave should be maintained at high degree, to ensure that any deterioration of an error rate of the main signal as kept as small as possible, and therefore, the above-mentioned service channel can not be used. Accordingly, a DSC system in which all digitized signals, including the order signal, are included in the main signal and are transmitted, is used.
There are four kinds of signals that can be transmitted by the DSC, i.e., the DSC synchronizing signal, the system switching control signal, the order-wire signal, and low speed data. The order-wire signal and low speed data are necessary in both the intermediate repeater station and the terminal office, but the system switching control signal is necessary only in the terminal office.
The system switching control signal is used to prevent problems in the main circuit and to maintain the quality of the main signal, and is the most important among the signals transmitted by the DSC.
On the other hand, in the signal processing portion in the intermediate repeater station, as mentioned already, the order-wire signal and the low speed data are taken out only after all of the other DSC signals are taken out. Therefore, if a fault occurs in this portion, a problem arises in that the system switching control signal can not be repeated to the receiving terminal office, and thus problems arise in the operation of the main circuit.